Various devices and methods have been provided heretofore for the purpose of obtaining liquid samples from below the surface of a body of liquid. These devices fall generally in respective ones of five groups.
The devices of the first group employ a container which is lowered into the body of liquid while in its open position and is provided with a trigger for closing the container by spring action, the trigger being actuated by a dropped messenger weight which trips the trigger whereupon the container is closed. Samples taken by these open devices are subject to possible mixing of the well fluids and contamination of the sample on passing through the air-liquid interface.
The devices of the second group employ a bottle-like container provided with a cork-like closure which is extricated by a jerk of the retrieval rope or line. These devices require auxiliary weights to sink them below the air-liquid surface, are limited in capacity and depth capability, and may involve undesired mixing of the fluids in the body of liquid when employed in a well.
The third group includes bailers having a single valve at the bottom of the container which is opened by bumping the bottom of the well. This device is intended primarily for removal of trash from the bottom of a well and is not suitable for the taking of true samples.
The fourth group comprises simple bucket-type devices, open at the top. They require weight to overcome flotation, and involve a mixing of liquids due to passage through the air-liquid interface zone. These devices do not provide depth-of-sample selectivity.
The fifth group includes devices having a chamber closed at the bottom by a control valve and open at the top through a sampling passage and having a freely movable valve member inside the chamber having a specific gravity less than that of the fluid to be sampled. The chamber is charged with gas to a pressure substantially the same as that in the fluid to be sampled at the sampling depth and the free member is held against the sampling passage which is held closed thereby. When the device is lowered in the fluid to be sampled, the passage is opened by release of the member when the pressures in the chamber and in the surrounding fluid are equal. The chamber then fills and is removed with the samples contained therein.
While the devices of these five groups may be suitable and effective for some sampling applications they are not effective for all purposes and, accordingly, it is an object of the present invention to provide an improved fluid sampling device which is effective for taking a truly representative sample of a liquid at a selected location below the surface of a body of the liquid.
It is another object of the present invention to provide an improved device which is effective for securing samples of liquid at selected depths without contamination by fluids or foreign matter present in the liquid through which the device must pass to a selected sampling position.